This invention relates to the radiographic diagnostic art and more particularly to a radiographic apparatus and method with video processing of x-ray shadowgraph images. The invention will be described with particular reference to x-ray diagnostic equipment with digital video processing. However, it will be appreciated that the invention has broader applications in other fields in which a television camera is operated intermittently to produce only a single or small number of frames of video data.
Previously, others have devised a radiographic apparatus for producing video images of x-ray shadowgraphs of a part of an examined object. See by way of example U.S. Pat. No. 3,573,461 which issued Apr. 6, 1971 to S. A. Ohlsson, U.S. Pat. No. 3,582,651 which issued June 1, 1971 to M. P. Siedband, U.S. Pat. No. 3,784,816 which issued Jan. 8, 1974 to S. Abrahamsson, or U.S. Pat. No. 3,848,130 which issued Nov. 12, 1974 to A. Macovski. In such radiographic apparatus, an x-ray source irradiates the examined object with radiation. A fluoroscopic screen converts the radiation which has traversed the object into an optical image. A television camera monitors the optical image and converts it into a plurality of frames of a video signal or image. An image processor is provided for processing the frames of the video signal to produce one or more video images for display on a video monitor.
Over the first several video frames, the amplitude of the gray scale portion of the video signal does not vary linearly with the intensity of light received from the optical image. Specifically on the first frame, a low optical intensity of light produces a very small gray scale signal. On the second frame, the same low intensity of light produces a slightly higher gray scale. Similarly over the first half a dozen or so video frames, the same low intensity of light produces progressively higher amplitudes in the gray scale signal. This transient nonlinearity in the response of the television camera to light, particularly at low levels, greatly impairs the value of the first half a dozen or so frames of the video signal. Accordingly, in the prior art radiographic devices, it has been common practice to discard a sufficient number of video frames to insure that the television camera has reached its steady state operating mode. The discarding of the first several video frames occurs every time the camera starts converting an optical image into a video signal.
This is undesirable because it increases the amount of radiation to which the object is subject. Each time the camera is actuated, the patient or object must be irradiated for an extra duration sufficient to produce the several discarded frames of data. In many instances, only a single or small number of frames of data are used to record an image in each actuation of the television camera. Thus, discarding the first several frames from the television camera may increase the patient's or object's dosage by a factor of three or more. Further in each radiation study, it is common to take several images of the examined area. The radiation source and television camera are turned off between taking each image.
One reason for the transient non-linear properties of the television camera is the capacitive properties of the target and the resistive properties of the electron beam. By way of brief review, light impinging on the target frees electrons which migrate to the inside surface of the target. The capacitive nature of the target permits the collected charge to be held by each incremental area of the target. As the beam sweeps each incremental area of the target, it causes the charge to be released and the potential across the capacitive incremental area to be discharged. The discharging of the potential across each incremental area releases a current from the target which is converted to the gray scale portion of the video signal. Under steady state operating conditions, discharging the potential across incremental areas of the target is substantially linear. However, the relatively high capacitance of the target and the relatively high resistance of the electron beam both cause the potential across an incremental area to approach zero exponentially. When the camera is first actuated, low intensity signals cause a potential which is so low that it is in the exponential region, which causes a non-linear response. After several frames, the potential across the incremental areas accumulate to a magnitude which is in the linear range.
Another reason for the transient nonlinearity of the television tube, is the nature of its target material. Commonly, the target has a large number of electron traps. When the target first receives light, some of the freed electrons fill the electron traps. This reduces the number of electrons available to produce the gray scale portion of the video signal. Particularly for low light intensities, the time required to fill substantially all of the traps and move the target into its linear responsive region, may be several frames.
In the unrelated field of television broadcasting, the obverse of the problem has been recognized. That is, when a received bright image is discontinued and the television camera views only a black or lightless field, the small accumulated potential decays exponentially to zero and the electrons filling the traps are gradually freed over several video frames. This causes a bright portion of the video field which suddenly becomes black to fade over a plurality of frames. The result is ghosts or other undesirable artifacts. To eliminate these artifacts, video broadcasters illuminate the target at a low level of optical radiation continuously while the television camera is operating. This maintains the accumulated potential in the linear range and the electron traps substantially full even when an area of the target is black. By combining an offset signal with the video gray scale signal, the constant light produced by the low level light source can be removed from the video signal. The camera's sensitivity to very low levels of intensity is, however, reduced.